1. Field of the Invention
This invention relates to a method of and an apparatus for directional induction logging for obtaining information of formation by measuring electric characteristics of formations by making use of a borehole. The invention is applicable to very extensive fields of utility. Among these fields of utility, those, to which the application is particularly desired, are as follows.
(1) Survey and exploring of petroleum and geothermal reservoirs:
The invention permits imaging of the circumferential distribution of the electric conductivity at depths of boreholes, and is thus effective for the survey and exploring fracture reservoirs and like anisotropic reservoirs. PA1 When carried out as a resistivity sensor in the MWD, the invention permits monitoring of upper and lower formations during horizontal drilling or the like. It is thus possible to drill a borehole without departing from a desired formation. PA1 The invention permits imaging of the circumferential distribution of the electric conductivity at depths of boreholes and is thus effective for the survey of anisotropic foundations. PA1 A. A magnetic field is generated by supplying an alternating current to the transmitting coil 31. PA1 B. The generated magnetic field generates an eddy current in a formation 8 around the tool such that the current is substantially proportional to the electric conductivity of the formation 8. PA1 C. The eddy current thus generated generates a magnetic field secondarily. This secondary magnetic field induces a voltage across the receiving coil 32 within the tool. PA1 D. The induced voltage is proportional to the magnitude of the eddy current, i.e., the electric conductivity of the formation 8 surrounding the coils. Thus, it is possible to measure the electric conductivity of the formation 8 by measuring the voltage across the receiving coil 32. PA1 E. Across the receiving coil 32, a voltage is also induced by mutual induction with the transmitting coil 31. This signal, however, is 90 degrees out of phase with respect to the voltage induced in the receiving coil 32 by the eddy current in the formation 8. Thus, this signal and the induced voltage can be separated from each other by use of a phase sensitive detector.
(2) Measurement sensors in the MWD (Measurement While Drilling) techniques for making measurements during drilling a borehole:
(3) Survey of foundations for the civil and construction purposes:
2. Description of the Prior Art
FIG. 25 basically shows an induction logging apparatus in the prior art, which is composed of transmitting and receiving coils 31 and 32 disposed coaxially in a borehole 7. As indicated in FIG. 2, borehole axis 18 is coincident with axis 19 of the coils. The principles underlying the induction logging apparatus are as follows.
Referring to FIG. 25, designated at a reference numeral 6 is a cable, at a reference numeral 20 a power source, at a reference numeral 23 a recorder, at reference numeral 33 a transmitting/receiving electronic circuit, at reference numeral 34 a winch, and a reference numeral 35 a pulley.
An induction logging apparatus which is actually used at present, has pluralities of coaxially arranged transmitting and receiving coils. However, the basic principles are the same.
Such induction logging apparatus with coaxially arranged transmitting and receiving coils are disclosed in U.S. Pat. Nos. 1,819,923, 1,913,293, 2,220,070, 2,220,788, 2,582,314, 3,067,383, 3,166,709, 3,226,633, 4,481,472 and 4,513,376 or the like.
In the induction logging apparatus described above, the sensitivity is fixed in the circumferential direction with respect to the hole axis. Therefore, the apparatus does not have any directivity in the circumferential direction. This means that with the Prior art induction logging apparatus, no information could be obtained about the distribution of the electric conductivity in the circumferential direction of the borehole.
Meanwhile, there have been proposed various induction logging apparatus for specific purpose, for instance, those disclosed in U.S. Pat. Nos. 3,014,177, 3,187,252, 3,510,757, 3,561,007 and 3,808,520.
In any of such special purpose induction logging apparatus, however, an orthogonal coil arrangement is used, and the magnetic field is rotated electrically or mechanically for measuring anisotropic extensions and inclinations of inclined and horizontal formations. Therefore, unlike the present invention, it is impossible to obtain the circumferential electric conductivity distribution around a borehole and obtain imaging of the distribution.
As a substitute for prior art electrodes mounted on dip meter pads, an induction type logging apparatus having asymmetric sensitivity is known, as disclosed in U.S. Pat. No. 3,539,911. In this apparatus, at least two electrically serially connected transmitting coils are disposed in an inclined fashion in a pad, and a receiving coil shaft is disposed between the adjacent transmitting coils such that the shaft is parallel to the hole axis. This apparatus, therefore, is irrelevant in the technical concept to the present invention.